feat(purchase): implement PurchaseController with CRUD operations and request validation

app/Domains/Purchase/Resources/PurchaseItemResource.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace App\Domains\Purchase\Resources;
+
+use App\Domains\Catalog\Resources\ProductVariantDefinitionResource;
+use Illuminate\Http\Request;
+use Illuminate\Http\Resources\Json\JsonResource;
+
+/**
+ * @mixin \App\Domains\Purchase\Models\PurchaseItem
+ */
+class PurchaseItemResource extends JsonResource
+{
+    /**
+     * @return array<string, mixed>
+     */
+    public function toArray(Request $request): array
+    {
+        $variant = $this->variant;
+        $product = $variant?->product;
+
+        return [
+            'id' => $this->id,
+            'cantidad' => $this->cantidad,
+            'precio_unitario' => $this->formatMoney($this->precio_unitario),
+            'total' => $this->formatMoney($this->total),
+            'product' => $product === null ? null : [
+                'id' => $product->id,
+                'nombre' => $product->nombre,
+                'slug' => $product->slug,
+            ],
+            'variant' => $variant === null ? null : [
+                'id' => $variant->id,
+                'nombre' => $variant->nombre,
+                'slug' => $variant->slug,
+                'definitions' => ProductVariantDefinitionResource::collection($variant->definitions),
+            ],
+        ];
+    }
+
+    protected function formatMoney(float|int|string|null $amount): ?string
+    {
+        if ($amount === null) {
+            return null;
+        }
+
+        return number_format((float) $amount, 2, '.', '');
+    }
+}